1. Field of the Invention
This invention relates to a polyolefin resin composition for metallized films. More particularly, it relates to a polyolefin resin composition for metallized films, which is superior in high-impact properties, easy heat-sealability as well as adhesive properties of metallized membrane and printability onto metallized surface and does not cause inferior taken-up figures such as "wrinkles", "take-up protuberance", etc. in taken-up films at the time of film making as well as at the time of metallizing processing.
In recent years, crystalline propylene-.alpha.-olefin copolymers composed mainly of propylene such as ethylene-propylene random copolymer, ethylenepropylene-butene-1 copolymer, propylene-butene-1 copolymer, etc. have been widely used for film use applications, mainly for general packaging films, laminates, etc., utilizing their characteristics such as superior transparency, high-impact properties, heat-sealability, etc. On the other hand, metallized films obtained by depositing metals onto plastic films in vacuo have been widely used for metallized yarns, architectural materials, packaging films, etc., utilizing their superior decorative properties, gas barrier properties, light-shielding properties, etc.
Particularly, aluminum-metallized film has been used in a large amount centering around packaging use applications, but those obtained by metallizing conventional, commercially available propylene-.alpha.-olefin copolymer films are weak in the adhesive force between the base film and metallized membrane, and extremely low in the printability and adhesive properties onto the metallized surface; hence it has been impossible to use them for use applications where printing, laminate, etc. have been required, and this has constituted a serious obstacle to their use development. Japanese patent application laid-open No. Sho 55-52333/1980 discloses that the cause of these difficulties consists in that a portion of hydrochloric acid-catching agents, slip agents, antioxidants or the like added into polypropylene film migrates or is transferred to the metallized surface to thereby deteriorate printability and adhesive properties. The present inventors have further investigated the cause of the deterioration of printability and adhesive properties in more detail, and as a result have found that fatty acid derivatives such as higher fatty acid salts, fatty acid amides, fatty acid esters, fatty acid amine derivatives have a worst influence, and that a part of waxes or antioxidants of a low molecular weight which are liable, to migrate or volatilize, also cause similar difficulties; hence usable additives have been extremely restricted.
Particularly, fatty acid derivatives such as higher fatty acid salts e.g. calcium stearate, sodium stearate, etc. used for neutralizing the acidic component of catalyst residue contained in polymers, higher fatty acid amides e.g. oleic amide, stearic amide, erucic amide, ethylenebisstearamide, etc. conventionally used as slip agents for films, even when added in a slightest amount of about 0.01% by weight, cause the wetting index to lower down to 33 dyn/cm or less and also make printing or adhesion onto the metallized surface impossible. However, these additives have so far been conventionally used as indispensable additives for polypropylene film, and polypropylene containing no such additives raises various problems at the time of film making or film post-processing. For example, unless the above-mentioned slip agents are added, slip characteristics or anti-block properties of films are extremely reduced, and taken-up films have wrinkles or a local enlargement of film roll i.e. the so-called take-up protuberance is formed, whereby productivity is reduced to a large extent. Particularly in the case of propylene-o-olefin random copolymers, these phenomena notably appear due to their low rigidity and great adhesive properties. This not only reduces the productivity and yield of films, but also,even if only portions free from wrinkles or take-up protuberances are selected and subjected to metallizing, wrinkles or protuberances are formed during the take-up step after the metallizing, whereby the productivity has been reduced. The lower the melting points of propylene-.alpha.-olefin copolymers, the more notable the phenomena. Further, the more the width of the films are broadened or the more the thickness of the films are reduced, the more notable the phenomena. This applies to films prepared by blending a grafted polypropylene obtained by graft-polymerizing anhydrous maleic acid or the like onto polypropylene (see Japanese patent application laid-open Nos. Sho 50-61469/1975 and Sho 55-52333/1980). Thus, such phenomena have constituted a serious obstacle to production of a metallized polypropylene film which has superior high-impact properties and heat-sealability as well as superior printability and adhesive properties of the metallized surface.
The present inventors have made various studies of a process for producing a metallized film which is superior in the adhesive properties of metallized membrane and the printability and adhesive properties of metallized surface and also superior in the film-making properties and processability into film and still retains characteristics of propylene-.alpha.-olefin copolymers having superior high-impact properties and low temperature heat-sealability, and as a result, have found that when there is used a composition obtained by blending a specified polyethylene in a specified amount, with a specified propylene-.alpha.-olefin copolymer, it is possible to obtain an objective superior metallized film.